In the field of fiber optic communications, it has been known for some time to increase the capacity of an optical communications link by propagating wavelength-division multiplexed (WDM) optical signals along optical fibers. Specifically, a WDM signal is composed of a plurality of distinct wavelengths of light, each such wavelength carrying a respective optical information signal, also known as an information "channel". The number of wavelengths (i.e., information channels) in a WDM signal is a system parameter and usually ranges from 2 to 128 (in the case of "dense" WDM, or DWDM).
As the WDM signal travels through an optical network, it gradually fades and must be amplified at various points along its route. Because of the large number of wavelengths that can be carried by a single WDM signal, and also due to the high data rates of the individual information channels, amplification is best performed by purely optical means.
Suitable candidate components for achieving optical amplification are rare earth doped-fiber amplifiers, such as erbium doped-fiber amplifiers (EDFAs). In an EDFA, it is possible to set the total output power level of the amplifier to a desired value, thereby to establish a relationship between the power of a WDM input signal entering the EDFA and the power of a WDM output signal exiting the EDFA. This also establishes a gain relationship for the information channels passing through the EDFA.
Ideally, the gain applied by the EDFA to the information channels remains at a constant value. However, transients caused by the instantaneous addition or removal of one or more individual optical information channels by an upstream add-drop multiplexer (ADM) or other device will affect the power of the WDM input signal. Although the EDFA instantaneously reacts to such a change in input power by providing a corresponding change in output power, thereby maintaining a constant gain for a brief amount of time, a natural recovery process is initiated soon thereafter by the EDFA, whereby the specified total output power eventually becomes redistributed among the new number of wavelengths (or information channels).
Clearly, in the case of a channel being dropped by an upstream ADM, the effect of this transient will be an eventual increase in gain for the remaining information channels as the EDFA settles into steady-state operation. Conversely, in the case of an optical information channel being added by an upstream device, the specified total output power becomes shared (possibly unevenly) among the now larger number of wavelengths, leading to a decrease in gain for the information channels as the EDFA reaches a steady state.
In order to suppress transients and thereby maintain a constant gain for the information channels, it would appear plausible to monitor the power of the WDM input signal and that of the WDM output signal, calculate the ratio of the two and keep the ratio at a constant value via a feedback mechanism involving the specifiable output power level of the EDFA.
However, this approach neglects the fact that in addition to carrying the information channels, the WDM input and output signals carry one or more optical service channels (OSCS) and the WDM output signal further contains amplified spontaneous emissions (ASE), from which it follows that the aforementioned ratio poorly represents the gain of the information channels alone. This results in poor transient suppression and errors in gain control, leading to a degradation in the quality of the information channels reaching downstream components of the fiber optic network.